FIG. 17 shows a conventional heat pump hot water supply system of an indirect heating method. In the conventional heat pump hot water supply system, a heat source device 20 having a heat pump refrigeration cycle for circulating a refrigerant as shown in FIG. 17 (indirect heating method) is connected with a water circuit 21 through a water heat exchanger 202 (acting as a condenser). FIG. 17 shows an example where the water heat exchanger 202 is contained in a housing of the heat source device 20. However, the water heat exchanger 202 may be connected with the heat source device 20 outside the housing of the heat source device 20. The water circuit 21 is configured such that hot water or an antifreeze solution heated by the refrigerant passing through the water heat exchanger 202 circulates in the water circuit 21. A circulation pump 24, a hot water storage tank 25, a radiator or fan coil unit 26, a floor heating appliance 27 and so on are connected to the water circuit 21 with pipes.
As methods of supplying heat to the hot water storage tank 25, a direct heating method and the indirect heating method shown in FIG. 17 are available.    (1) In the direct heating method, hot water to be used for hot water supply is directly heated by the water heat exchanger 202.    (2) In the indirect heating method, heat is supplied by heat exchange between the hot water or antifreeze solution heated by the water heat exchanger 202 and hot water to be used for hot water supply, the heat exchange being effected through a second heat exchanger 29 provided in or outside the hot water storage tank 25.
(Indirect Heating Method)
Referring to FIG. 17, the indirect heating method will be described. In the indirect heating method, as shown in FIG. 17, the hot water or antifreeze solution heated in the water heat exchanger 202 can be directly supplied to a radiation heating appliance such as the radiator or fan coil unit 26 or the floor heating appliance 27 and can be used for heating a room. In the indirect heating method, as shown in FIG. 17, heat supply channels are switched by a three-way valve 23. During a water heating operation in the hot water storage tank 25, the three-way valve 23 is set such that the water circuit 21 is switched to a channel in which the second heat exchanger 29 is placed. Channel switching may be effected by using a plurality of two-way valves. As a result of channel switching, the hot water heated in the water heat exchanger 202 flows into the second heat exchanger 29. The water temperature in the hot water storage tank 25 rises by a heat quantity heat-exchanged with this hot water that has flowed in. When a value measured by a tank water temperature sensor 35 provided in the hot water storage tank 25 or on a wall of the hot water storage tank 25 reaches a water heating temperature setting value, the water heating operation is finished. When a user starts using hot water supply, the hot water is discharged from an upper portion of the hot water storage tank 25. The discharged hot water is mixed with water. The mixed hot water is supplied to the user as the hot water at a specified temperature set by the user through a remote controller or the like. On the other hand, the water is supplied from a lower portion of the hot water storage tank 25. Thus, the inside of the hot water storage tank 25 is always fully filled with water.
(Direct Heating Method)
In a water heating operation by a conventional heat pump hot water supply system of a direct heating method (for example, Patent Document 1), operation at the beginning is performed at a compressor frequency computed in accordance with the temperature of hot water supplied from a hot water storage tank to a heat source device. Then, when operation of the heat source device has stabilized, a deviation of a hot water outlet temperature of the heat source device from a hot water outlet temperature setting value is computed and the amount of change in this deviation is computed. Then, based on the obtained deviation and amount of change in the deviation, a correction amount of the compressor frequency is obtained. The current operating frequency is corrected with the obtained correction amount, and operation is performed at the corrected compressor frequency. In the conventional heat pump hot water supply system of the direct heating method, the compressor frequency is controlled as described above. In the direct heating method, low-temperature water at a lower portion of the tank is always supplied to the heat source device, so that operation can be performed with stable heat exchange efficiency in the water heat exchanger 202.